if I change my email, how can I go back to see what's in my old email and folders

What change on your email are you thinking about?

If you change the email address, you will have a different email account, so your old email and folders will remain the same and new mail using your new email address will go to the new email address account.

You will have to choose and log onto the email account that you want to use at the time.

If you only change the password for your current email account, then nothing will change except that you will have a different password. All of your old email and folders will remain the same.

Or, are you thinking about changing something else? If yes, what would that be?